A variety of optical attachment systems comprising plural lens elements have been disclosed in the prior art for taking short distance shots. These attachment systems are attached to a main camera lens on the side of an object to be photographed. For fixed-focus type, main lenses, Japanese Laid Open Patent Application H03-39917 gives examples of attachment lens systems which consist of three lens elements. These include, in order from the object side, biconvex, biconvex, and biconcave lens elements, or a single biconvex lens element and two coupled lens elements that, when joined have a convex surface on the object side and a concave surface on the image side. Japanese Laid Open Patent Application H11-101939 describes a configuration comprising three lens elements which are, in the order from the object side, biconvex, biconcave, and biconvex. A problem in Japanese Laid Open Patent Application H03-39917 is that the useful diameter of the first lens element must be increased in order to enable wide-angle shots when a single biconvex lens element and two joined meniscus lens elements are used. Attachment lenses for those fixed-focus type, main lenses as in the prior art described above are not suitable for zoom-type main lenses because the fluctuation of aberrations becomes too large with zooming.
For zoom-type main lenses, Japanese Laid Open Patent Application H05-224121 discloses a three-lens-element attachment lens that has a concave surface nearest the object side, and is formed of two joined meniscus lens elements and a biconvex lens element. With this configuration, the attachment lens enables the camera to take short distance shots. However, a problem occurs in that lateral color aberration may significantly degrade the image at wide angles. Recently, image-detecting devices which are mounted on a television or video camera tend to employ CCDs (Charge-Coupled Devices), and high resolution is required over the entire detection surface. Therefore, the attachment lens for photographing, as described above, is required to provide optical performance of high resolution over the entire specified image field.
In the prior art described above for zoom-type main lenses, the first lens element surface, which is concave on the object side, must have a strong curvature. Accordingly, the coupled surface, which is the second surface, tends to have stronger curvature. Therefore, the center thickness of the coupled lens elements, especially the first lens element, becomes larger, thereby causing the lens weight to increase.
The object of the present invention is to provide an attachment lens for short distance shots which is suitable for taking pictures or capturing electronic image data of objects, even when used with zoom-type main lenses, and has high resolution optical performance over the entire specified imaging field while being kept lightweight and compact.